1. Field of the Invention
This invention relates to a semiconductor device having a circuit or an element comprising thin film transistors (hereinafter referred to as xe2x80x9cTFTsxe2x80x9d) on a substrate having an insulating surface, and to a manufacturing method thereof. More specifically, the present invention relates to electro-optical devices (called also xe2x80x9celectronic equipmentsxe2x80x9d) typified by a liquid crystal display device including a pixel section (pixel matrix circuit) and driver circuits disposed around the pixel section and formed on the same substrate and, and electrical devices (called also xe2x80x9celectronic appliancesxe2x80x9d) having the electro-optical device mounted thereto. Note that, throughout this specification, the semiconductor device indicates general devices that can function by using semiconductor characteristics, and that electro-optical device, semiconductor circuit, and electronic equipment are all categorized as semiconductor devices.
2. Description of the Related Art
Development of a semiconductor device having a large area integrated circuit, that comprises TFTs formed on a substrate having an insulation surface, has been made progressively. An active matrix type liquid crystal display device, an EL display device and a close adhesion type image sensor are typical of such semiconductor devices. Particularly because TFTs using a polycrystalline silicon film (typically, a poly-Si film) as an active layer (the TFT will be hereinafter referred to as xe2x80x9cpoly-silicon TFTxe2x80x9d) have high electric field mobility, they can form a variety of functional circuits.
In the active matrix type liquid crystal display device, for example, an integrated circuit that includes a pixel section for displaying images for each functional block, a shift register circuit, a level shifter circuit, a buffer circuit each being based on a CMOS circuit and a sampling circuit, and so forth, is formed on one substrate. In the case of the close adhesion type image sensor, a driver circuit such as a sample-and-hold circuit, a shift register circuit, a multiplexer circuit, and so forth that drive the pixel section, are formed by using the TFTs.
These driver circuits (which are also called xe2x80x9cperipheral driver circuitsxe2x80x9d) do not always have the same operating condition. Therefore, the characteristics required for the TFTs are naturally different to certain extent. The pixel section comprises a pixel TFT functioning as a switching device and an auxiliary storage capacitor, and a voltage is applied to a liquid crystal to drive it. Here, it is necessary to drive the liquid crystal by alternating current, and a system called xe2x80x9cframe inversion drivingxe2x80x9d is widely applied. Therefore, one of the required characteristics of the TFT is that an OFF current value (a drain current value flowing through the TFT when it is in the OFF operation) must be sufficiently lowered. In a buffer circuit on the other hand, because a high driving voltage is applied, the TFT must have a high withstand voltage such that it does not undergo breakdown even when a high voltage is applied. In order to improve the current driving capacity, it is necessary to sufficiently secure the ON current value (the drain current value flowing through the TFT when it is in the ON operation).
However, the poly-silicon TFT involves the problem that its OFF current is likely to become high. Degradation such as the drop of the ON current value is observed in the poly-silicon TFT in the same way as in MOS transistors used for ICs, or the like. It is believed that the main cause is hot carrier injection, and the hot carriers generated by a high electric field in the proximity of the drain presumably invite this degradation.
An LDD (lightly doped drain) structure is known as a structure of the TFT for lowering the OFF current value. This structure forms an impurity region having a low concentration between a channel forming region and a source or drain region to which an impurity is doped in a high concentration. The low concentration impurity region is called the xe2x80x9cLDD regionxe2x80x9d.
A so-called xe2x80x9cGOLD (gate-drain overlapped LDD) structurexe2x80x9d is also known as a structure for preventing deterioration of the ON current value by hot carrier injection. Since the LDD region is so arranged as to overlap with a gate wiring through a gate insulation film in this structure, this structure is effective for preventing hot carrier injection in the proximity of the drain and for improving reliability. For example, Mutsuko Hatano, Hajime Akimoto and Takeshi Sakai, xe2x80x9cIEDM97 Technical Digestxe2x80x9d, pp.523-526, 1997, discloses a GOLD structure using side walls formed from silicon. It has been confirmed that this structure provides by far higher reliability than the TFTs having other structures.
In a pixel section of an active matrix type liquid crystal display device, a TFT is disposed for each of dozens to millions of pixels and a pixel electrode is disposed for each TFT. An opposing electrode is provided on an opposing substrate side beyond a liquid crystal, and forms a kind of capacitors using the liquid crystal as a dielectric. The voltage to be applied to each pixel is controlled by the switching function of the TFT. As the charge to this capacitor is controlled, the liquid crystal is driven, and an image is displayed by controlling the quantity of transmitting rays of light.
However, the accumulated capacity of this capacitor decreases gradually due to a leakage current resulting from the OFF current, or the like. Consequently, the quantity of transmitting rays of light changes, thereby lowering the contrast of image display. Therefore, it has been customary to dispose a capacitance wiring, and to arrange another capacitor (called a xe2x80x9cstorage capacitorxe2x80x9d) in parallel with the capacitor using the liquid crystal as the dielectric in order to supplement the capacitance lost by the capacitor using the liquid crystal as the dielectric.
Nonetheless, the required characteristics of the pixel TFT of the pixel section are not always the same as the required characteristics of the TFT (hereinafter called the xe2x80x9cdriving TFTxe2x80x9d) of a driving circuit such as the shift register circuit and the buffer circuit. For example, a large reverse bias voltage (a negative voltage in n-channel TFT) is applied to the gate wiring in the pixel TFT, but the TFT of the driver circuit is not fundamentally driven by the application of the reverse bias voltage. The operation speed of the former need not be as fast as that of the latter.
The GOLD structure has a high effect for preventing the degradation of the ON current value, it is true, but is not free from the problem that the OFF current value becomes greater than the ordinary LDD structures. Therefore, the GOLD structure cannot be said as an entirely preferable structure for the pixel TFT, in particular. On the contrary, the ordinary LDD structures have a high effect for restricting the OFF current value, but is not resistant to hot carrier injection, as is well known in the art.
For these reasons, it is not always preferred to constitute all the TFTs by the same structure in the semiconductor devices having a plurality of integrated circuits such as the active matrix type liquid crystal display device.
The present invention is a technique for solving the above stated subjects, and an object of the present invention is to enhance the operation performance and the reliability of a semiconductor device by optimizing the TFT structure disposed in a circuit or an element of the semiconductor device, correspondent to its function. Further, the present invention provides a manufacturing method for materializing such semiconductor device.
In order to solve the above stated problems, the present invention has a goal for example in allotting an optimized structure of TFT according to the function required by a circuit or an element formed in a semiconductor device such as a liquid crystal display device. Namely TFTs of different structures exist on a same substrate.
In concrete, it is preferable that an element which imposes great importance on sufficient reduction of OFF current value (switching element etc.) has a TFT structure which places more importance on reduction of OFF current value than operation speed. On the other hand, a TFT structure which places greater importance on increase in operation speed and on prevention of degradation due to hot carrier, a noticeable problem which arises at the same time, is preferred in an element which has its major object in high speed operation (driver circuit element etc.)
The present invention enables improvement of the operation performance and the reliability of a semiconductor device by properly using the above stated TFTs over the same substrate.
A measure is further taken in the structure of the LDD region of an n-channel TFT for preventing the degradation due to hot carrier injection. In other words, the present invention has a characteristic in disposing a concentration gradation in the LDD region between the channel forming region and the drain region so that the concentration of n-type impurity element becomes gradually higher as getting closer to the drain region. This structure calculate upon higher effect of relieving the electric field.
Further in case of disposing the above stated concentration gradation, the concentration of n-type impurity element included in the proximity of the boundary between a LDD region and the channel forming region of an n-channel TFT becomes lower than the concentration included in the proximity of the boundary between the LDD region and the drain region. A higher effect of relieving electric field is obtained in the similar way.